Error 'System.Data.DataSet' has base class or interface

Question

Hi Guys
i have a dll and i tried to disassemly it and i have .net reflector
i did it and i have the code this dll with  .net framework 1 and when i build the dll with no changes i have this error

Referenced class 'System.Data.DataSet' has base class or interface 'System.Xml.Serialization.IXmlSerializable'

defined in an assembly that is not referenced.

You must add a reference to assembly 'System.Xml'.

can any one answer me how i can solve this problem and what is the problem with this dll

thanks in advance